This invention relates to a chuck table which is preferably used in holding a semiconductor wafer in a semiconductor wafer machining device, such as a semiconductor wafer dicer.
As is well known among people skilled in the art, a chuck table including a pedestal having a circular depression formed in its upper surface, and a disk-like chuck plate positioned in the depression of the pedestal is widely used as a chuck table for holding a semiconductor wafer in a semiconductor wafer machining device, such as a semiconductor wafer dicer. The pedestal is formed from a suitable metal such as stainless steel, and the chuck plate is formed from a porous material such as porous ceramic. An additional depression of a circular shape is formed in the center of a bottom surface of the depression of the pedestal, and communicates with a suction path. The inner diameter of the depression of the pedestal and the outer diameter of the chuck plate are substantially the same. A suitable adhesive, such as epoxy resin, is coated in a peripheral edge annular area in a lower surface of the chuck plate and/or a peripheral edge annular area, which surrounds the additional depression, in the bottom surface of the depression of the pedestal. The chuck plate is positioned and pressed in the depression of the pedestal to fix the chuck plate in the depression of the pedestal. In an outermost peripheral edge portion in the bottom surface of the depression, a recess is formed for accepting a surplus portion of the adhesive fluidly moved when pressing the chuck plate. After the chuck plate is fixed in the depression of the pedestal, the upper surface of the pedestal and an upper surface of the chuck plate are ground, whereby the upper surface of the chuck plate, and the upper surface of the pedestal surrounding the upper surface of the chuck plate are made coplanar sufficiently precisely.
The semiconductor wafer is usually mounted via a mounting tape in an opening formed in the center of a mounting frame. By gripping and transporting the frame, the semiconductor wafer mounted on the frame is positioned on the chuck plate in the chuck table. Then, a suction source, such as a vacuum pump, is actuated to suck air through the chuck plate, the additional depression, and the suction path, whereby the semiconductor wafer is attracted onto the chuck plate.
According to the present inventors"" experience, the foregoing conventional chuck table poses the following problems: After repeated use, there is a tendency that the chuck plate is raised slightly (e.g., by several tens of microns) relative to the pedestal, so that the upper surface of the chuck plate, and the upper surface of the pedestal fail to be coplanar sufficiently precisely. Machining to be applied to the semiconductor wafer, such as dicing, usually involves positioning of a machining tool, such as a cutting edge, and the semiconductor wafer relative to each other, with the upper surface of the pedestal as a reference. Thus, the rise of the chuck plate relative to the pedestal means that the semiconductor wafer cannot be positioned sufficiently precisely relative to the machining tool. Moreover, the rise of the chuck plate is not uniform in the entire circumferential direction. Hence, the upper surface of the chuck plate tends to be slightly inclined, starting in an initial state, and the semiconductor wafer attracted to the chuck plate also tends to be inclined. Such inclination of the chuck plate also makes it impossible to position the semiconductor wafer sufficiently precisely relative to the machining tool.
A principal object of the present invention is to provide a novel and improved chuck table which does not raise a chuck plate relative to a pedestal after repeated use.
The inventors of the present invention conducted extensive studies. As a result, they found the following cause of the rise of the chuck plate relative to the pedestal: The surplus of the adhesive, which has flowed into the recess formed in the outermost peripheral edge portion of the depression of the pedestal and has cured there, expands owing to wetting and/or heating during repeated use of the chuck table to apply an upward force to the lower surface of the chuck plate. Based on this finding, the inventors found that the above-mentioned principal object could be attained by forming the recess for accepting the surplus adhesive in a lower end portion of a side surface of the depression, rather than in the bottom surface of the depression. When the recess is formed in the lower end portion of the side surface of the depression, the surplus adhesive that has flowed into and cured in the recess applies the force to the chuck plate not at the bottom surface of the chuck plate, but at its outer peripheral surface, even if the surplus adhesive expands during repeated use of the chuck table. The direction of the force is radially inward of the chuck plate. Therefore, the rise of the chuck plate relative to the pedestal can be prevented fully reliably.
To prevent the rise of the chuck plate due to expansion of the surplus adhesive that flowed into and cured in the recess formed in the bottom surface of the depression of the pedestal, it is conceivable to increase the depth and/or the width of the recess formed in the bottom surface of the depression. However, this measure would decrease the strength of the pedestal markedly, or would make it necessary to form the pedestal of a large size.
As a chuck table for solving the above-described technical challenge, the invention provides a chuck table including a pedestal having a circular depression formed in an upper surface thereof, and a disk-like chuck plate positioned in the depression of the pedestal, the chuck plate being fixed in the depression of the pedestal by means of an adhesive interposed between a bottom surface of the depression-and a lower surface of the chuck plate, wherein
a recess for accepting a surplus of the adhesive is formed in a lower end portion of a side surface of the depression of the pedestal.
In preferred embodiments of the invention, the inner diameter of the depression of the pedestal and the outer diameter of the chuck plate are substantially the same, and an upper surface of the chuck plate and the upper surface of the pedestal are substantially coplanar. The recess is shaped like a ring extending continuously in a circumferential direction in the lower end portion of the side surface of the depression. A plurality of the recesses may be formed with spacing in the circumferential direction. Preferably, the chuck plate is formed from a porous material, an additional depression of a circular shape is formed in the center of the bottom surface of the depression of the pedestal, the additional depression communicates with a suction path, and the adhesive is interposed between a peripheral edge annular area in the bottom surface of the depression, which surrounds the additional depression, and a peripheral edge annular area in the lower surface of the chuck plate, whereby the chuck plate is fixed in the depression of the pedestal.